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Why does static directional stability decrease with altitude?
At constant CAS, high altitude means lower air density and therefore higher TAS: this decreases the aerodynamic damping during a rotation.
For this reason, I cannot understand why directional stability should decrease with altitude, as stated in this chapter:

Shouldn't the restoring (yawing) moment resulting from a sideslip be bigger when the aerodynamic damping is less effective?
EDIT:
Guys, thank you so much for your answers.
I should have provided some more background about this subject.
I am an ATPL student, and I came across this question while preparing for my Principles of flight exam:

The only informations I could find on the book are the chapter I posted above and this:

I could not find any reference on the book -and on EASA learning objectives for the ATPL theory- about the Mach-related effects on directional stability.
The chapter above, as you said, is quite confusing.
The Cn-B graph shows how strong the initial tendency of the nose to yaw towards the relative wind is, "closing" the angle of sideslip: the steeper the slope of the curve, the quicker the tendency of the A/C to re-align with the airflow. If I understand correctly, the graph doesn't tell anything about the dynamic behaviour of the A/C, it's all about static (directional) stability.
So, I cannot really understand why aerodynamic damping (or some other effect related to an increase of altitude, other than compressibility) should flatten the slope as the book -and the question- seem to suggest.
This is a super fascinating topic -as everything related to Principles of flight- so I would love to have a deeper understanding of it, not just because I have to pass the exams.
By the way, if you have any book/source to suggest it would be amazing.

The aerodynamic damping link is dependent on the aerodynamic forces. What happens to the magnitude of aerodynamic forces with increasing altitude?

@ROIMaison The aerodynamic damping is dependent on the rotation (tangential) velocity about an axis and inversely proportional to the True Airspeed of the aircraft.
At constant CAS (constant dynamic pressure), the magnitude of aerodynamic forces doesn't change for a given AoA, am I right?

The way I read it, the indicated section talks about high altitude (= low-density) flight, and notes that aerodynamic damping is low because the aerodynamic forces on the rudder are low. The text doesn't mention constant CAS.

Aren't we talking about dynamic stability here?  Static margin is unchanged, as the vertical aerodynamic center is at the same spot relative to the CG, so static stability should be the same.  The effect is on dynamic behaviour.  The reduced damping is mostly because restorative forces decline with air density while inertial forces remain the same.

@JohnK  Uuuhh - that (restorative forces decline with air density) from you! What declines are the motion-induced forces because TAS grows with altitude. Restoring forces stay constant because (as you correctly say) the vertical aerodynamic center is at the same spot relative to the CG and dynamic pressure stays constant.

Yeah bad choice of words.  I guess "reaction forces generated by displacement of the body" would be better descriptive?

@JohnK, Displacement is better but fails to hint at the motion and would also cover a change in $\beta$. It is really the motion-induced part which changes with density. So the best is "reaction forces generated by motion of the body".

I don't think the person who wrote the question in attachment IC-081-050 understands the topic of lateral stability well enough to be qualified to judge others on it. Also, the text you quote jumps between static and dynamic stability without making clear each time what is meant with the term "stability".
Definitions first: Static stability concerns the forces and moments that oppose a change of state. Dynamic stability concerns the forces and moments that oppose a movement. In both cases higher stability means higher forces and moments. The difference is in their cause.
From now on, I focus on lateral stability only. Static stability first: A typical change of state is flight into a lateral gust: Suddenly, all of the aircraft flies at a changed sideslip angle. If we neglect niceties like that the tip of the fuselage encounters the gust before the tail does, the effects are:

Destabilizing c$_{n\beta}$ contribution of the fuselage which is independent of Mach or angle of attack since the center of pressure of the fuselage is well ahead of the center of gravity.
Stabilizing c$_{n\beta}$ contribution of the wing which is proportional with aspect ratio, sweep angle and dihedral and grows with the square of the lift coefficient.
Strongly stabilizing c$_{n\beta}$ contribution of the vertical tail which initially is constant but deteriorates with higher sideslip when the tail reaches and exceeds its maximum side force and stalls like a wing. This contribution grows slightly with an increase in the subsonic Mach number but drops significantly with higher dynamic pressure due to elastic deformation of the tail and the fuselage it is attached to.

Next, what is meant by higher altitude? Your first sentence tells us it is flight at higher TAS but the same CAS and, hence, the same dynamic pressure and lift coefficient but a higher Mach number. c$_{n\beta}$ should increase ever so slightly from the higher side force slope on the tail but due to the higher TAS, any excursions from a straight flight path should grow proportionally to it. I guess that those very noticeable higher excursions are the reason for the author of attachment IC-081-050 to think that  c$_{n\beta}$ shrinks with altitude, but that is wrong.
Flying faster at the same altitude will significantly decrease c$_{n\beta}$ due to elastic deformation, but if CAS is kept constant, this effect will not be present. It could also be that the author of attachment IC-081-050 confuses the effect of higher dynamic pressure on c$_{n\beta}$ with that of higher TAS at higher altitude, but that is equally wrong.
A third reason for the wrong impression that c$_{n\beta}$ changes with altitude is that due to the higher flight speed the same gust will cause a smaller change in sideslip angle at higher altitude. But the coefficient is not c$_{n\;gust\:speed}$, but c$_{n\beta}$, so for being different, a different yawing moment change must be caused by the same change in sideslip angle at different altitudes, which is wrong.
Now on to dynamic stability: A typical movement is expressed as the  yaw rate $r$. Dynamic yaw stability is determined by yaw damping. As you correctly say it decreases with higher TAS and in many jets this requires the use of a yaw damper for flights at higher altitude. Because dynamic stability has its own coefficient (c$_{nr}$), this has nothing to do with the slope of c$_{n}$ over $\beta$. For dynamic stability the yawing moment coefficient must be plotted over yaw rate which is clearly not the case in attachment IC-081-050. Of course, it is also possible that the author has confused static and dynamic stability and attributed the altitude dependency of the latter to the former. But this would be a really bad mistake to make.
Also, the paragraph highlighted in yellow from your text is, while not outright wrong, at least misleading. Only if you interpret the slope the author talks of here as the slope of c$_{n}$ over $r$ will I agree with all he or she says. The context, however, implies he or she means the slope of  c$_{n}$ over $\beta$ which would be wrong.

Does the X-15 wedge tail have anything to do with this?  In very thin air at very high speeds, does the shock wave actually blank the stabilizer behind it?  Will the effect (on damping) be noticeable at high subsonic. (Is choice 3 the right answer?)

@RobertDiGiovanni The wedge was chosen because it would create more drag to fair the blunt trailing "edge" than to cut off the tail at the thickest point. But that is really only helpful in hypersonic flight and makes very little difference at subsonic speed. The lever arm of the drag force is small (tail length times sin($\beta$)). It should not be the reason for a change of c$_{n\beta}$ with density.

@PeterKämpf thank you very much for the detailed answer.
I believe that the author of the question simply wants us student pilots to state that dynamic stability decreases with higher TAS, but posted the wrong graph. We are required to comply with EASA so called "learning objectives", and one of them (posted in the picture above) is about the effects of altitude on dynamic stability.
I don't know why they provided the cnβ graph attached to the question when they are clearly asking about dynamic stability.
Thank you again for your help!

@Andrea, I had great difficulty answering my pilot theory exam (while having PhD on the subject) exactly because of such issues. Every second question (or offered answer) was wrong in one way or another. One needs to translate the question to 'what they actually wanted to say'. And what they (pilots/instructors) know is that 'flying is harder at high altitudes'. Why? Obviously, 'stability' is lower! Now let's find a graph about stability...

@AndreaAppiani I have long lost all hope for adequate understanding in officialdom of the topics they administer. I work on an IoT thingy with a small Li-Ion battery and am required to lie to my customers, because I must state that tampering with the battery will cause fire (it will not, not enough energy) and is prohibited. At the same time I must give detailed instructions how the battery can be removed for safe disposal. Go figure! –

@PeterKämpf really appreciate efforts to get terms and explanations correct (especially for teaching aides!).  It seems to boil down to:  damping (drag) helps corrective forces resist instability and damping resists corrective force overshoot, resulting (in both yaw and roll) in less change on there respective axes and fewer oscillations during re-stabilization.

@RobertDiGiovanni Yes, understanding the terms is important, especially once you put all of them into a simulation and expect it to give realistic results! I compare $c_{n\beta}$ with the stiffness of a spring (actually the product of $c_{n\beta}$ and dynamic pressure) while $c_{nr}$ can be compared to drag, say, of the bobbin of a spring pendulum in a viscous fluid. $c_{n\beta}$ and inertial forces produce an oscillation and $c_{nr}$ makes it die down. Inertial effects are the coefficient of the second derivative and $c_{nr}$ of the first in the differential equation of motion.

We've got a rather complex "can of worms" here.  Some of the quoted material seems to confuse cause with effect, or to confuse static effects with dynamic effects.

Shouldn't the restoring (yawing) moment resulting from a sideslip be bigger when the aerodynamic damping is less effective?

It seems like you are imagining that with less damping acting in opposition to yaw rotation, the restoring moment from sideslip will somehow be able to act more quickly or more effectively to introduce a yaw rotation to end the sideslip.
That's not a correct way to view the situation.
To understand why, you have to realize that the root cause of aerodynamic damping of yaw rotation is that the yaw rotation automatically creates a difference in sideslip angle between nose and tail.  The whole fuselage can't all be experiencing zero sideslip.  If the tail (vertical fin) "weathervanes" into alignment with the local airflow, the nose will be experiencing a sideways flow that generates a yaw torque that opposes the yaw rotation.  Similarly, if the rudder is used to keep the nose aligned with the local airflow, the vertical fin will be experiencing a sideways flow that generates a yaw torque that opposes the yaw rotation.  And note that the steeper the positive slope of the curve shown in the textbook, the stronger this opposing yaw torque will be.

At high altitude and high TAS the aerodynamic damping on the rudder is less effective, so directional stability decreases shown by a reduced positive slope.

That's not really a correct way to view the situation either.  A reduction in directional stability ("reduced positive slope") really can't be explained as a result of "damping". A reduction in directional stability ("reduced positive slope")   could be the result of effects related to high Mach, but it's not completely clear that this is in fact what the quoted text is trying to describe.
The truth is that whenever we talk about "damping", we need to specify exactly what mean to say is being "damped". Generally we are talking about a dynamic effect--something related to the rate of something else.  So is the quoted passage trying to address aerodynamic damping of the rotation rate in yaw, pitch, or roll?   Or is it trying to address damping of an oscillation, such as the pitch phugoid, or such as the "Dutch roll" oscillation?  Or are we actually talking about a reduction in the "weathervane" yaw torque per degree of sideslip (i.e. the static directional stability) for a given IAS-- which would not be a conventional usage of the term "damping"?  The quoted passage fails to clarify this.
It's true that some oscillation tendencies tend to be most pronounced at high altitude, for a given CAS or IAS, but the quoted passage hasn't adequately explained why this should be so.
Increased static directional stability would be expected to cause increased damping of yaw rotation rate.  It's confusing to suggest that increased damping of yaw rotation rate causes increased static directional stability, and decreased damping of yaw rotation rate causes decreased static directional stability, as the quoted passage seems to do.
There are at least two different reasons why an aircraft tends to be more prone to various dynamic oscillations at high altitude (and therefore high TAS), for a given IAS--
One, strong damping of yaw or pitch rotations tends to reduce "Dutch roll" or pitch "phugoid" oscillations, respectively.  And for a given IAS and G-load, a higher TAS will be associated with a larger radius (smaller rate) of curvature of the flight path, so the stabilizing effect of yaw or pitch damping will play less of a role in the aircraft's dynamics.
And two, at high Mach numbers the slope of the static directional (yaw) stability curve is indeed reduced.

Thank you very much, in the meantime I edited the question to add some more background.

at high Mach numbers the slope of the static directional stability curve is indeed reduced - proof, please! With Mach the sideforce-over-sideslip curve becomes a bit steeper, making the tail more effective. What leads to the reduction at higher Mach is elastic deformation from higher dynamic pressure. Please don't confuse the two!

@PeterKampf -- looking into it.  So, you are essentially saying that the only reason supersonic fighters tend to feature much larger vertical tails than their 1940's piston-driven counterparts, is because of elastic deformation?

@quietflyer No, the question is about subsonic flight and the supersonic airplanes need the larger tails because of thinner air on the upper side and the reduced supersonic lift curve slope - all purely supersonic effects. Elastic deformation has its share but is not alone.

Shouldn't the restoring (yawing) moment resulting from a sideslip be bigger when the aerodynamic damping is less effective?

No - damping is a force proportional to velocity, in this case yaw rate $\dot{\beta}$. The restoring moment is a function of yaw angle $\beta$, which will be smaller at lower air density.
Once the aeroplane starts responding to the restoring moment by moving back towards neutral, the damping starts playing a roll, with lower damping generating faster return to neutral and more overshoots.

At high altitude and high TAS the aerodynamic damping on the rudder is less effective, so directional stability decreases shown by a reduced positive slope. This can lead to an increased tendency to Dutch roll with increasing altitude.

Reduced damping:

cannot cause a reduced positive slope, since one is a function of yaw rate, the other a function of yaw angle.
does cause increased tendency to Dutch roll with increasing altitude.

The dynamic pressure is eliminated from the moment coefficient $C_{n \beta}$ and the argument that reduced dynamic pressure causes reduction of the coefficient slope does not make sense.

The coefficient is c$_{n\beta}$, so the sideslip angle is the basis, not gust strength!

@PeterKämpf With a constant $C_{n \beta}$, dynamic pressure reduces at increasing altitude. All aerodynamic entities change, mass does not, yet you state that dynamic pressure is of no interest, only the dimensionless coefficient?

Reduced density! Dynamic pressure stays the same, hence same IAS but higher TAS at altitude.

After much thought and credit to other responders ...

why does static directional stability decrease with altitude?

It will not, if aircraft stays below critical Mach number, although increased stagnation pressure on the forward fuselage relative to other surfaces should be considered (even 300 knots is pretty fast!)

damping on the rudder is less effective, so directional stability decreases

Yes, but not directly.  Damping has nothing to do with directional stability.  In the simplest sense, imagine a gold pendulum under water, at sea level, and on top of Mt. Everest.  Viscous drag limits the overshoot of each swing by limiting the velocity, therefor angular momentum of each swing. All three pendulums eventually stop.
The correcting force (gravity) is the same in each case if each pendulum starts from the same place.
With swept wing large airplanes, like airliners, the lack of damping manifests into instability because, at a certain degree of Velocity and deflection, overshoot becomes self-reinforcing and the yaw-roll cycle gets worse and worse.
At altitude, lift requirement is the same, therefor the lift differential (of the swept wing) for a certain degree of yaw is the same.
Because of lower damping, the velocity of the roll is faster.  This couples with the yaw created by drag differential (of the wings).
The sideslip adds fuel to the fire with each yaw-roll cycle.

@Andrea Appiani perhaps the highlighted portion is only saying that stability is lost because with less damping (drag against motion), the aircraft will yaw more for a given input.  The key words may be sideslipping to the right, which indicates greater spiral instability.  I have seen much confusion here because (especially with swept wings) of dihedral effect, a yaw to the right produces a skid to the left, which rolls the plane to the right.  So we see a  loss of damping at higher altitudes in roll as well.  Appreciate (as an ATP) you are trying to understand all this a bit better.


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On a Diamond DA-42, under what circumstances will the propeller auto feather?
I will be doing a multi-engine course in a 2006 Diamond DA-42 soon and I'm curious if this is true for the aircraft and under what circumstances the propeller would auto feather. For example, if the engine is starved of fuel or fails for whatever reason, would it auto feather? How about if the governor fails, and if so, under what types of failure with respect to that system would it auto feather? Or maybe the ECU has a fault and the system doesn't automatically switch to ECU B for that engine without pilot input. The POH doesn't specify if this aircraft will do this or not for any sort of scenario.
I am aware that turning the engine master off will cause the governor valve to open and the oil to flow out, resulting in the pitch becoming coarser/higher and feathering, during which time the accumulator will close to maintain pressure to unfeather when it's needed (engine master turning back on). I am also aware that if the RPM is below 1300, then feathering isn't possible due to the coarse pitch stops.

The DA-42 does not have Autofeather. Autofeather means that the prop will automatically  feather when a a loss of thrust is sensed. There may be some mechanical abnormalities that would cause the prop to feather, but that is still not Autofeather.

By auto feather, I didn't mean "autofeather" / a system on board that tells the propeller to feather under certain circumstances. I meant, what failures would cause the propeller to result in a feathered position. I should've written that better.

There is no autofeather function per se on a DA-42 - and by that I mean analogous to an autofeather system that you would find in a larger or turboprop powered airplane.  Automatic feathering of the propellers will occur if an engine is shut down or fails, for whatever reason, at an engine speed in excess of 1300 RPM. A prop governor failure, however, will not cause the propeller to feather, nor will it cause it automatically shut down. See the emergency procedures section of the AFM. As to the ECUs, the system should automatically detect a failure of the active ECU and switch over to other ECU.  If this does not occur, then the pilot may manually select the other ECU using the voter switches on the left side of the instrument panel.

Automatic feathering does not occur if the engine fails.  Feathering is accomplished by shutting down the engine with the appropriate engine master switch.

If the propeller governor fails, isn't that what is keeping the pressure in the prop hub and keeping the blades from feathering while flying?

No, if the governor fails, the propeller will not feather.  The service manual says a loss of propeller oil pressure will feather the propeller, but it isn’t clear if they mean an abnormal loss of pressure, or a reduction in pressure due to the normal operation of turning off the Engine Master Switch. I have no experience with the DA42 but when I read the Flight Manual and the Service Manual, it seems ambiguous. I guess that is why you are also asking about it.

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Who is the manufacturer of the power inverter used by Ampaire electric aircraft?
Please see the picture within this article.

It's probably a device from Cascadia, Unitek, Sevcon, or Emsiso.
The motor looks like an Emrax axial flux motor. At one point, Emrax's user guide said their motors are compatible with controllers from the four manufacturers I listed above. Now, the user guide just links to a broken URL.
Cascadia used to be Rinehart Motion Systems. Cascadia's controllers are more expensive, but their documentation and support is excellent. I would highly recommend their systems.

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DIY ultralight aircraft in Germany
I want to build for myself an ultralight aircraft as it's my dream project since I was young. I plan to build the aircraft as a 10 years project so I want to plan out everything and know if it will be even possible first.
I live in Germany so I'll follow the European aviation regulations. The specs for the aircraft are: 200 kg MTOW, 120 kg empty weight, 80 km/h cruise speed, 50 km/h stall speed, 90 km/h max speed, 10 m wingspan, the aircraft use an electric turbofan engine provide 7 hp thrust. Currently, with the wind tunnel test I got L/D ratio of 15.22. Combine these elements give my aircraft estimated performance of 1.08 m/s climb rate and cruise range of 100 km.
So my question is: will this aircraft be suitable to fly in Germany? Since I've been here for quite a while, I understand how complex the German laws are. So I would also like to ask: if it's possible to register a complete self-build aircraft? My flight plan is quite simple, a local flight in VFR for recreational purpose. So the aircraft will fly mainly within unregulated airspace as for experimental aircraft. But my worry is also if it's possible to even register this aircraft for flying at all. So if anyone familar with this topic, I would appreciate the help.
Edit: So for the commends and the answer I recieved, I found many ask why I use a turbofan instead of a propeller. The aircraft I want to build will resemble a fighter jet, the F-117N Seahawk, that's why I want to mount the engine internally. At first I actually want to use a jet engine, but given the budget limit and the problem come with thermal and strutural of an ultralight, I decide to go with 2 turbofans for total of 12hp(I decide to add some margin for safety to the engine rated power of 7hp). Yes, I understand this design is very complex, will not as efficient as already established design and potentially not as good as I expected, but it's my dream project and if it meet at least the safety factor and have good enough performance, then I'll take it. I tried with many methode to increase the performance like: sharkfin winglets, blended body design, mainly use composite material like carbon fiber laminate,... which should help the final design performance to match the small prototype design I tested in CFD and wind tunnel.

Related: Are there experimental aircraft regulations in Germany? From the question it seems LTF-UL applies for MTOW < 600kg

@mins thanks! yeah, as I though, it's as complicated as I expect it will be. I might try to ask my laywer, I'm not sure he will know about this topic though. thanks for the help

A 10 meter biplane might be interesting, and maybe a propeller rather than a turbofan.  7 hp may give marginal performance, but 2 of them may make for an excellent recreational aircraft.  You need to be able to climb in downdrafts.

Is this an aircraft that you're also designing yourself, that you'll then build, or is the design from someplace established? The challenges involved in designing an aircraft are far beyond the reach of a first-time DIY project. The challenges in building from an established design are significant, but surmountable, given the right resources and guidance.

the aircraft I would like to built would resemble a fighter jet, particularly the F-117N seahawk, that's why I want to use turbofan as I can put them internally. with current setup I can have range of 100km at 80km/h cruise speed and moderate maneuvers which fit my requirement for local flight as I only fly for fun, not much for travel. I understand that it will be lots of work to build an ultralight, but it's my dream and I've calculated the budget, the only problem is the legal part. With advice from mins and the answer I recieved, if the laws checked out, then build it is't a problem.

Do you mean that the cruise is 80 knots, not km/h?  Eighty km/h is only 43 knots.

@DaveGremlin Hi, sorry for late answer, I haven't check the question for a while. so no, I really mean 80km/h, the aircraft I want to build will mainly be a cruiser and not much of a sport aircraft. With my current wing configuration, it generates 2003N while generate around 131N of drag. Combine that with my current engine(7hp) and battery power, I'll have 100km range for cruise, which is good enough as I only perform a local flight. I would like to know what you mean by 80 knots? Since with 7hp, I don't think I can pull as high as 80 knots.

@SairextIrkaris  I was surprised by the slow airspeed.  A Piper Cub cruises at around 65 knots, at 43 knots you'll be flying slower than a flexwing.  What sort of V speeds are you expecting for the whole aircraft (not just the wing), especially VROT and VS0?

@DaveGremlin the current wings I have have VS1 of 40km/h with flap 10 so VS0 would be 45km/h and VROT would be 60km/h. To achieve this the wing have quite high CL and pretty large control surface. The 10m wingspan, tapered wing design and winglets will reduce the induced drag somewhat. with the current 7hp engine I can pull VH around 54 knots before the airspeed cause the propeller in the fan to loss efficiency and blade stall.

@SairextIrkaris  Sorry, I'm missing something, you're making an aircraft with the shape of an  F-117 (which is optimised for flight around 600 knots) and a wingspan of 10 metres (of which 3-3.5 metres will be fuselage contributing a lot of induced drag at low speed) and you expect it to stall at 24 knots?  How much will this aircraft weigh?

@DaveGremlin I think you missed the 200kg MTOW. the aircraft will be relatively light with carbon fiber composite and Ti grade 5 alloy for the reinforcement. it's an inspiration from the F-117N shape, but it won't be exact, the swept wings is the first thing I change which changed to tapered swept wings with small swept angle of 5 degree. it still resemble the F-117N but it won't be the same size or weight, it's like a smaller bird with similar look, although that does cause quite a bit more parasitic drag due to sharp edge.

Get in touch with the Oskar Ursinus Vereinigung (OUV). They have experts who will guide you through the process. The full name of that association adds its purpose: "Deutscher Verein zur Förderung des Eigenbaus von Luftfahrtgerät e.V." (German Association for the Promotion of Self-construction of Aircraft)
The association supports work on self-built aircraft in a variety of ways. The OUV is financed by membership fees and donations, and self-builders can become members.
The OUV is:

a recognised assessor of self-built aircraft,
advises the self-builder and prepares the 3 expertises which you will need during the process,
is a recognised noise measurement centre
and mediates between the self-builder and the Luftfahrt-Bundesamt in Braunschweig.

It's best to contact them before you start - every aspect of the work on your design needs their oversight, so adding them only later in the process complicates things.
Your specs seem reasonable, except for the "electric turbofan". If you choose electric propulsion, please make best use of the little energy your 120 kg budget allows and fit a large, slow-turning propeller.

Thanks for the advice about the propulsion system, but I already decided to follow my dream project so it will resemble a fighter jet, the F-117N seahawk. the turbofan I use can be put internally, it provides enough thrust for a comfortable level flight and ~1m/s climb. with this design it will not be efficient as the L/D ratio is 15.22 which is quite low mainly due to the swept wings for aestetic which need surface area to conpensate. but it fits my requirements as I only fly local flight and want to experience flying feeling, so I'm fine with that efficiency if it looks like what I dream of.

@SairextIrkaris If you decide on such an inefficient platform, the propulsion will not make much of a difference anymore. However, then I doubt that the 12 hp will be sufficient.

that's one of the problems I have with the design I chose, currently I'm still trying to fine a way to go around it, with the battery I have I can supply my engine 15kW or around 20hp which is sufficient for the 2 engines at max power. A rule of thumb I've heard is 100W/kg, so mine may not be enough. It's the challenge to go with this design and I'll see what I can do to overcome that. For now I'll clear the legal part first, then I'll try to fix the problem later after I know it's possible to get a airworthiness licence. thank you for the advice.


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Why does F-18 have low wing sweep?
Compared to most other fighters, the f-18 has a low sweep angle on its wing; 20 degrees compared to the f-16s 40 degree sweep and f-15s 45 degree sweep. Additionally the wing has a relatively high aspect ratio.
At first, I thought it was related to the requirements for taking off from a carrier, but the prototype YF-17 was originally designed as a land based fighter, and had a similar wing.
What are the performance trade-offs for this kind of wing on a fighter jet?
Edit: also is the sweep angle the reason the wing is mounted further back compared to something like the English Electric lighting?

This could get complicated pretty fast. I'm not saying that because I know all the answers. Just that you ask questions that may have a lot of overlapping reasons.
The fighter could have been designed as land based while still having that 20 degree wing sweep(it was). Northrop had used that style of wing since at least the prototype N-156 which became the F-5s and T-38. The big difference in later Northrop aircraft and their respective prototypes are the much more prominent leading edge root extensions(LERX) providing better low speed stability and controllability.
What are the trade-offs? Less wing sweep usually limits top speed on the aircraft because of how the supersonic shock cone changes as speed increases, but does offer better low speed performance. It could also, in isolation, be worse at high speed maneuverability, but so many other factors play into this that saying so would be too reductive.
The low sweep is certainly a factor in why it is mounted so far aft, but may not be the only reason. Maybe the designers wanted good downward visibility from the cockpit. Maybe they had to elongate the fuselage to fit certain systems. The heart of it is probably as you said though. By shifting the wings further aft you shift the aerodynamic center aft and you typically want the aerodynamic center aft of the center of gravity to keep the plane stable.


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Why aren’t there Seaplane AWACS?
Why are there no seaplane AWACS?
AWACS aircraft are excellent force multipliers, with their detection and networking capabilities. Due to their altitude, they can allow a naval force to “see” beyond the radar horizon. However, they are often limited to operating from a country's land bases, or in the case of the US and France, from large, expensive aircraft carriers.
Since building carriers that can operate them is expensive, why don’t more navies opt to use seaplanes, which can be supplied by a destroyer, or amphibious assault ship. That would provide a fleet with radar cover regardless of its location relative to land bases.

seaplanes only work in small calm lakes, rivers, and sheltered very flat harbours (they should be called "lake planes" !

The E-2 Hawkeye is a carrier based aircraft that fills that role for the US Navy.  It is an older airframe, but very capable, and a replacement is in work.
https://en.wikipedia.org/wiki/Northrop_Grumman_E-2_Hawkeye

Seaplanes don't do particularly well on the rough seas of the open ocean, and the Navy abandoned that idea a long time ago.  The Hawkeye is perfectly capable of providing support to all ships and aircraft within a Battlegroup.

The US Navy is just one of many navies in the world, though.

@VladimirFГероямслава, indeed it is, but I don't know enough about each of the other Navies to answer for them.

Seaplanes need extensive support infrastructure, and are hard to operate in all but the smoothest of sea states.
They're also hell to maintain because of the salt water corrosion affecting everything (ship borne helicopters and aircraft have similar problems but to a lesser degree).
That's why most countries did away with them when land bases became prevalent enough to use those instead and the range of land based aircraft enough to be able to operate further from your base.

Because we are no longer in the age of battleships.
In the age of battleships, your major firepower came directly from ships (big guns), and those ships would have benefited greatly from AWACS or similar for targeting enemy forces. Modern ships have cruise missiles and other smarter weapons for long-distance firepower. It is a different world.
How are AWCS "force multipliers"? By supporting land, air and sea forces.
If you have significant land forces but no air forces in a particular battle zone then you either capture an airstrip and use that, or you rely on modern ships that may not have that much firepower but do have advanced radar and command/control systems which can support your troops.
If you have significant air forces in the area then a seaplane-based AWACS isn't needed unless the rest of your forces - fighters, bombers, etc. - are also seaplanes. And no modern armed forces have any significant seaplane fighters, bombers, etc. You fly the AWACS out of the same friendly air fields or aircraft carriers as the rest of your aircraft.
If you have significant naval forces but no aircraft carriers then you rely on shipboard radar and command/control systems. You may also be able to make use of helicopter-based AWACS systems, as many ships that are smaller than aircraft carriers do have helipads.

There is also a political element to this question.
If a nation has no political desire to use force away from their own territory, they can simply use land based aircraft operating from their own runways.
If they intend to use force in support of allies, they can reasonably assume that ally will allow use of runways, given anywhere that can operate airliners can support a useful number of AWACS very quickly (vs the equipment needed to support useful numbers of combat aircraft in terms of apron space, ground support, weapon handling etc). Getting political permission to operate unarmed airliner conversions is also much easier and faster than weapon carrying platforms.
If a nation intends to use force offshore and already owns carriers, placing AWACS on those carriers is the logical and easy choice, or at worst support a land based AWACS on station using carrier based tankers.
For a nation that does not own full sized aircraft carriers but does have a domestic aviation industry, modifying an existing helicopter may be less capable but much easier than establishing a bespoke flying boat program.
This produces a fairly narrow slice of users interested in a water based fixed wing AWAC, none of whom have chosen to spend money in this way at this time. Unless a power with serious wealth and expansion ambitions emerges among the pacific nations (islands with sheltered water but limited space for runways) it seems unlikely anyone will invest the resources in developing one.

Most of the current AWACS models are either based on civilian aircraft (E-3, E-7), or have been adapted to other roles (E-2 -> C-2). The obvious advantages of this is that any R&D costs are spread over more airframes, and the military operators get lots of experience virtually for free from the civilian world.
If you were to build seaplane AWACS, what problem would it solve?
All the current US AWACS models are able to refuel in flight, meaning that the main limit would be the onboard crew. You could land and swap crews of course, but what location would be so far from land that this would be a more attractive option than returning to a land base to swap crews?
Seaplanes are a lot more sensitive to weather compared to land based planes, if the waves are too rough, they can't land. If they are unable to land, they need the range to go somewhere else: either a place far enough away that the weather is likely to be better, or to a land base. They also require more maintenance, as they are essentially big floating fragile metal things operating in a harsh environment with lots of salt water.
Point Nemo is the point on earth the furthest from any landmass, approximately 2.500 miles from Easter Island with its international airport. If it became necessary, I suspect that the US would be allowed to use that to base AWACS planes on. But: Why? A point far away from everything is unlikely to be of interest to anyone. If you have a fleet (or a convoy) transitioning through the area, it makes more sense to patrol the entire route rather than just flying around the immediate area where the patrol is located. And; AWACS are primarily meant for airborne threats, you still need other assets for surface- or submerged threats...

authors: Vladimir F Героям слава


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ADS-B - Measure distance from position on ground to an aircraft
I have a docking station for autonomous drones in the field. It is equipped with a Raspberry Pi, an 1090 MHz ADS-B stick, and it knows it's geometric position from a previously done precise measurement. This station needs to maintain a cylindrical "flight security zone" around itself. Whenever an aircraft breaches this zone, security measures are taken. Furthermore...

the zone's exact dimensions are not known yet, but we can assume a diameter of around 5km and a height of around 200-300m.
within the zone, the measurements of aircraft positions should be "as precise as possible". Outside the zone, I don't really care
the measurements need to work for every aircraft, optional messages only certain aircrafts send are not an option.

My Question in a nutshell
How can I measure the distance between an aircraft and myself, and what would be the most precise way to do it?
My ideas so far
I have used dump1090 to receive ADS-B messages from the stick, which returns them in raw- or SBS-1 format.
The horizontal distance from my station to the aircraft should be deductable in sufficient precision from the ADS-B messages, I guess. An SBS-1 message, for example, contains the already calculated WGS84 coordinates.
The vertical distance is a bigger problem. As I understand, ADS-B works with pressure altitude referenced to standard pressure. That leaves me with two options:

Integrate a barometer into my setup and correct the aircraft's altitude with reference to my local pressure.
ADS-B also mentions a message with GNSS height (Type Code 20-22), but I can't seem to find any info about when to expect it. Is is completely optional, is it mandatory below transition height...?

Are my general assumptions correct? Are there other possibilities?
Thanks in advance
Edit 1:
Note that if there are rare and "unusual" occurances where planes might not identify themselves via ADS-B, like military machines that don't use ADS-B for reasons of camouflage, or planes with broken transponders, then these occurances can be ignored. I realize that, with just an RPi and an ADS-B stick, my capabilities are limited. But whenever a plane (passenger or sport) or a helicopter (police, rescue...) comes by, I'd like to know it.

“the measurements need to work for every aircraft, optional messages only certain aircrafts send are not an option”. That effectively rules out ADS-B. Even in places where it’s mandatory there are still a number of exceptions, and of course there could be malfunctions. In which country/region are you?

As jcaron points out, this will not work. If you absolutely need an area completely free of air traffic, you need to work with the local aviation authorities to set up a (temporary) restricted area

We're talking about germany and I was under the impression that an ADS-B transponder is mandatory nowadays.  How do the authorities or airports know who's currently flying, do they have a different technology? Please also take note of my edit.

@user3792852 -- just for completeness you might want to google up the regs for gliders (sailplanes).  I'm not sure whether they are required to have transponders and ADS-B out in Germany or not.  Likewise ultralights or microlights.  I do understand that you know you will never cover all possibilities, but just something to consider.

In some airspace classes you can fly without any transponder at all (ADS-B or otherwise)

To properly close this topic, here is what I ended up doing:
Note, however, that this problably makes no practical sense. Since the area to be controlled is not a special airspace, aircrafts below 5,7 tons MTOW and 250 knots aren't required to use ADS-B. The messages I will mostly receive will be coming from airliners, which will be way above my area of interest (Correct me if there is anything wrong here).
I start dump1090 with the following parameters:

./dump1090 --net-only --interactive

Now I can open a TCP port on localhost:30003 and receive messages in SBS1 format. This is the lazy approach, as this format is easiest to parse.
Furthermore, I integrated a barometer into my setup. The SBS1 messages contain the pressure altitude according to the standard atmosphere in feet, so I measure my pressure altitude in feet using the following python code:
local_pressure = get_barometer_reading()
my_alt = 145366.45 * (1 - (local_pressure / 1013.25)^0.190284)

The formula to calculate the pressure altitude was taken from here.
The value my_alt is subtracted from the aircraft's altitude and I have the height difference.

I haven't checked your implementation, but this approach should work. In addition you could use the geometric height as well, but then you need to use a different format (SBS-1 didn't include that info IIRC).

But still, I won't recommend relying on ADS-B alone for the type of project you are doing. You may be able to get access to a stream of surveillance data from ATC for your area of operations. Can you tell more about the kind of operation you are planning to do, and what type of airspace it is in? I especially interested because I am professionally involved in integrating Unmanned Aviation in the airspace.

The airspace is uncontrolled and the drones will be operating at a height of 30-50 metres. Is there anything specific you'd like to know?

Thank you for getting back to me! What is the nature of the operation? Surveillance, transport, agriculture? Is it in urban area, near an urban area? Can you tell me where it is?

It will mainly be surveillance of larger industrial areas with little to no people underneath. Please understand that I cannot give any more details, as this would compromise security.

As you say, horizontal distance can be easily deduced from ADS-B messages.
ADS-B can use multiple different sources for position. If it's using GNSS (as most are), it should be transmitting its geometric altitude in addition to its barometric altitude. If it is using DME/DME, for instance, it will not report geometric altitude.
Your options for barometric altitude are to integrate a barometer or to do the same thing airplanes do and get a local altimeter setting. This could be done manually with a handheld radio or automatically if local METARs are available online.
Using both altitudes and taking the closest one is the most conservative approach. If only using one, you should use the barometric altitude and correct for non-standard pressure.
Keep in mind that Europe does not mandate ADS-B for light aircraft flying VFR, which are the aircraft most likely to be so low! So you absolutely cannot rely on ADS-B as your sole source of manned aircraft avoidance.

It's starting to look like ADS-B is only for large passenger aircrafts. As for medical-, police- or sport aircrafts, no one really knows where they are exactly when airborne. Everything just happens on sight. Is that about right?

@user3792852 Not just sight- most aircraft will at least have a mode S transponder so ATC and to some degree other aircraft will know where they are. With several receivers spread over a geographic area it's possible to triangulate such aircraft by timing measurements of their mode S responses. But this only works if the aircraft are in range of a radar facility- mode S responses are only sent in response to interrogations sent to them by a radar facility or other aircraft.

A certified and correctly functioning ADS-B transmitter sends position,  (usually GPS derived), pressure altitude and geometric height above WGS84 ellipsoid.
If you know your receiver's position and height (above ellipsoid, not MSL) and you can get the geometric height from ADS-B, then it is easy to solve your problem.
The geometric height is normally encoded as difference from pressure altitude. It is contained in the velocity messages. See EUROCAE ED102A or RTCA DO-260B for encoding details.
But beware! Many aircraft in German lower airspace are not equiped with ADS-B transmitters, and those that are equiped may not perform as advertised. Notable examples of non ADS-B aircraft are part of the Medical Helicopter and Police fleet, and those can be expected to fly low and fast.
Don't think that because you have an ADS-B receiver you can fly drones safely and legally. Flying autonomous drones is not allowed in Germany, as far as I know. Look at Germany's DIgital Platform for Unmanned aviation DIPUL for more information about regulations and maps of geozones.

Nothing is illegal if the authorities grant you permit for it. We're currently in the process of acquiring said permit, and it would help our cause if we had a means of surveilling the airspace around our drones.

In that case: I am really happy to hear that you are planning going down the legal road. I would not use an in-house developed surveillance solution based on ADS-B alone, but instead use a combination of official surveillance sources, perhaps combined with local additional sensors to fill gaps (depending on where you are planning to fly). If you can tell a bit more about your project I may be able to help you further.


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ID a 1930's-40's aircraft
This cribbage board is a mystery I am trying to solve.    It has an aircraft flanked by linked flags - they look like the Union Jack and the former Canadian Flag - and a maple leaf.  It suggests that it's a commemoration of a 30's/40's aviation link between those countries.  I have asked the question on every forum I can find, and sent requests to various aviation museums.  A number of aircraft has similar features - but none has the tail structure of this one.  Any input would be appreciated.  I understand that it may be a 'flight of fancy' on the part of the artist!
0
Thank you all for your input. The suggestion that the flag could be Australia or New Zealand ... I hadn't thought of that. The maple leaf could have been included because the flags were similar. Will hunt further on that one.
Re the suggestion of maybe 3 flags ... I see two flags, each with the fly draped over the centre. Will look further into that one.
Re the cribbage board itself - no markings. I unscrewed the top plate in the hope there might be something. No luck. And the green felt underneath it looks sufficiently aged to have been there from scratch.
Re the Fiat BR.20 Cicogna and Armstrong WW - yes, very close. They look to be wartime aircraft, with the nose bubble etc. Was there a civilian version? Re the Albatross - that was one of the first I found. Four engines knocked it out of the race.
I'm intrigued with the artwork. It has enough features of various aircraft to suggest that the artist was looking at an image - but that tail is different to any I've found. Maybe Albatross tail, Hudson or similar front. If the artist was looking at an image, why depart from that?? And the flags - it's commemorating something. The mystery deepens and the search continues.
Edit: the latest is that I am contacting Air Canada to see if it's part of their history.  A commemoration of an event, a PR item, maybe even available to passengers to play onboard.  Although with its weight, it is probably not something that would be carried on board.  Stay tuned!

The cribbage board fascinates me to some extent more than the picture of the aeroplane on the front. Is there any other markings/inscriptions on it at all that would help identify it?

The plane shares the two engines and tail with the Armstrong Whitworth Whitley, which in its civilian version was operated by the British Overseas Airways Corporation, but I cannot find any pictures of it in order to contribute an answer. Therefore, this becomes a comment.

Other twin engine with lookalike tail is Fiat BR.20 Cicogna

Based on a comment by community member "mins", I took a look around.  Mins initially thought it was the Lockheed Electra Model 10, but that didn't look quite right to me.  "Mins" later edited their comment to include the Model 14, so I performed some searching, and I think the cribbage board features a Lockheed 14-WF62 Super Electra.
If you look at the photo on this page on Airliners.net, you can see a striking resemblance to the aircraft on the cribbage board.
I'm not sure of the fair use of the photo on that page, so I'll just link to it here.
Most of the credit for this answer goes to "mins", because their suggestion is what helped me find this specific photo.
According to the caption on Airliners.net:

Sold to the first British Airways in August 1939 as G-AFYU and crashed off Malta on 21 December 1939.

The use by British Airways in the late 1930's could possibly explain the presence of the Union Jack on the cribbage board.

The tail shape of the Lockheed 14-WF62 Super Electra is wrong

You specifically asked about the tail structure which is very unusual. The closest match I believe is the prototype de Havilland DH.91 Albatross, and would have a more obvious British / Canadian connection than the Electra.
It first flew in 1937. Of course the major difference is that the Albatross has four engines: the engraver may have used it as a source, but realised four engines was too fiddly and resorted to two.

The prototype DH.91 Albatross, E-2/G-AEVV is first flown at Hatfield by chief test pilot R G Waight. source

Also mentioned here with some more speculations.

@kegaro and mins --interesting that Wikipedia has a photo of a (apparently the second) prototype with a very different tail -- https://en.wikipedia.org/wiki/De_Havilland_Albatross#/media/File:Albatross_1938_prototype.jpg . I think there's no question the first prototype as shown in this answer was a major source of inspiration for the artwork-- note the shape and placement of the vertical tails, and also the straight trailing edges and swept leading edges of the wings and horizontal stabilizer. However there's obviously been no attempt to fully replicate the plane in all details (eg # engines)

@kegaro and mins -- as to whether the choice of a twin-engine radial-engine design, and the fuselage shape and other fuselage details, were heavily inspired by the Super Electra, or is just the artists generic idea of what an airliner should look like, is hard to say.  One problem with the Super Electra idea is that the tops of the passenger windows should be lower than the bottom of the cockpit side windows, and the passenger windoes should be closer together.  But since some artistic license was obviously taken anyway, it's possible that the Super Electra was a significant inspiration.

I cannot provide a full answer, but I'm providing more clues if that can help a fellow poster:
Possible model
The aircraft seems to be a variant of Model 14 Super Electra , the Hudson, used during WW2 as a light bomber and maritime reconnaissance aircraft. The number of windows seems to correspond to Hudson Mk.I, the variant delivered to UK:

Hudson Mk.I (source).
The number of windows varies a lot for Electra. While Chamberlain returned from Berlin in the airframe above, G-AFGN had more windows at the time. He also traveled on another Electra for the trip to Berlin, with less windows. So relying on the number of windows might be risky.
Flags
The flags seem to be 3:

Uk, Canada, but also either Australia or New Zealand. Thus the commemoration could be related to some event with these three nationalities involved.
What to commemorate?

Amelia Earhart, the first woman to fly across the Atlantic on 1932, taking off with a Lockheed Vega, from Harbor Grace, Newfoundland, Canada. She disappeared while flying a Electra 10-E in 1937.

Anyone who wants to add clues is welcome, this post is a community wiki.

Having the maple leaf would suggest Canada to me. The upper flag could be a Red Ensign, used in Canada (and Australia) before the current national flags were designed. The Union flag below would also have been widely used in Canada in the inter-war period.

Some issues w/ the Super Electra idea are given in this comment https://aviation.stackexchange.com/questions/104828/id-a-1930s-40s-aircraft#comment292391_104842.  Another issue is the cockpit windscreen shape-- see how the Super Electra windscreen comes to a much sharper point on the aircraft centerline.  (The same could be said of the DH 91 Albatross, to a lesser degree.)
Honestly I think it's kind of a tossup as to whether the artist drew any inspiration for the fuselage from from the Super Electra specifically, or not.


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why does the bottom of a 737 - 800 engine look slightly flat?
The 737 - 800 is one of my favorite airliners and so I've always noticed that it's engines look a bit flat on the bottom, I haven't seen any other airliners do this so, is there any specific reason for this?

Image for reference ^ (edited)
https://www.istockphoto.com/es/fotos/boeing-737-800
Source of Image^

The nacelle is oval-shaped because the accessory gearbox, which is usually at the bottom of the fan case, has been moved on the side to increase the ground clearance, as visible in Where is the generator in a large turbofan of a commercial airliner?

The 737 was designed in the 1960s, when high-bypass turbofans didn't exist, so the engines were narrow, but inefficient. It was also designed to use built-in airstairs, for small local airports that don't have any ground stairs or jet bridges. This means the original 737 had to sit close to the ground.

To keep the regulatory advantages of an older design, there are some things Boeing can't change, and one of them is landing gear design. The 737's landing gear is a very simple system that can't be easily made significantly taller.

If you can't make the gear taller, and the engines keep getting larger, both for better efficiency because of the continuous supersizing of the plane - the Max 10 is 90% larger than the -100 Classic - then all that's left is to flatten the nacelle's bottom and/or place the engine forward of the wing rather than under the wing. There has to be some clearance between the engine and the ground, because the wings flex, and any less clearance than on the 737 NG or Max will result in the nacelles occasionally striking the ground on landing.
The -800 uses a flattened nacelle bottom and the MAX does both.

(source)


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What is this gas during taxi?
The other day I was on an A321 when I noticed a condensed gas coming out of the portion I  circled on the image here:

I've seen it happen several times but never understood the reason behind it. Can anyone explain what its purpose is?


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Why do commercial jets fly so high?
Everyone ever heard of a plane knows they usually go quite high into the atmosphere. To my knowledge, this is to help the plane go faster in low atmospheric densities and reduce aerodynamic drag. But is there any other reason for this? I feel there must be a few more as high altitude flights require pressure maintenance and much more powerful engines.

Could you be a bit more specific about what sorts of "other reasons" you have in mind, or why you think there should be some? As it stands, this doesn't sound like a very interesting question because it already contains the obvious answer.

Another aspect is that two engine airliners need to be able to fly on one engine, which means they always have more than twice the minimum power for flight available. The speed of sound means they can’t use this power to fly faster, but they can use it to fly higher- this is part of why the 787 or 737 cruise way higher than the A340 or 747

@TooTea "but other than the better customer experience, the lower personnel costs, the cost savings on fuel, and the better environmental impact... what has high altitude ever done for us?" :-)

They fly that high because they're too heavy to fly any higher.

One of the main reasons to fly high has nothing to do with efficiency and everything with passenger comfort: flying above the weather. The majority of weather phenomena, especially turbulence, happens in the first few km of the atmosphere. By simply flying higher, the ride is much smoother. Additionally, deviations or even cancellations due to the weather are much reduced

I think predominantly it is about efficiency though. A Boeing 737 true airspeed (TAS) at ~15,000 ft is ~280 knots and at ~35,000 is ~500 knots. Furthermore, in the tropics, the troposphere (where the weather is) rises to levels higher than typical commercial aircraft can fly

@tedioustortoise The majority of flights are not in the tropics though. And the question specifically asked about reasons other than lower drag.

If it was cheaper to transport passengers in dry tumblers I imagine budget airlines would...

@tedioustortoise You have the cart before the horse. The cruising altitude is set, then the aircraft is designed to be most efficient at that altitude.

Unfortunately, this popular and selected answer is Wrong.  Airlines are a business that operates on surprisingly thin margins.  They operate in a way to save fuel.  If it was just about weather, they would fly lower (saving energy of climbing) on good weather days and only fly high when weather and turbulence required it -- only flying high enough to avoid that day's weather.  Instead they fly at 36kft because that is where the efficiency benefit from climbing stops.  They fly here every day to save fuel every day.

Yes there is another reason. During the cruise phase of the flight the aeroplane burns fuel and its weight reduces, and its lift must therefore reduce as well to maintain equilibrium. So should it now:

reduce its AngleOfAttack to reduce lift;
lower its speed to reduce drag and lift;
or gain altitude to fly in even thinner air?

Above graph is from this answer which contains more details on the topic. It would be most efficient for the aircraft to continuously gain altitude as the weight reduces, but the highways in the sky have assigned altitudes so for safety reasons the aeroplane ascends in steps.

The most important reason for commercial aircraft to fly high is fuel efficiency.
The thrust specific fuel consumption (TSFC) of jet engines scales with the square root of the temperature.  Colder air means the engine can have a larger   temperature increase before melting the engine.  That is thermodynamically good.
So,
$\mathrm{TSFC}=\mathrm{TSFC}_0\,\sqrt{\theta}$
Where
$\mathrm{TSFC}_0$ Is the sea level $\mathrm{TSFC}$.
and
$\theta\equiv \frac{T_{t,0}}{T_{sls}}$
$T_{t,0}$ is the total temperature at the ambient condition the aircraft is flying at.
$T_{sls}$ is the sea level standard temperature.
The standard atmosphere model says that the ambient temperature gets colder until 11km (36,089 ft).  At that point, the temperature remains approximately constant until very high up -- then it gets warmer again.
Consequently, there is a fuel efficiency benefit to flying at 36,000 ft -- but not really in going higher.  You will find that most commercial aircraft cruise right around this altitude (given considerations for weather and traffic).

I think that historical perspective is the easiest way to understand why flying higher was needed in the first place (other than efficiency as requested in the question).
The first commercial airliner with a pressurized cabin was the Boeing 307 Stratoliner. One of the first operators was Transcontinental & Western Air (T&WA) and the development of the aircraft was carried out closely with it.
"Donald Webb Tomlinson at T&WA carried out five years of high altitude flight research, with a Northrop Gamma and a Douglas DC-1, which helped determine that T&WA would need a four-engine airliner with a pressurized cabin. A ceiling of at least 16,000 ft (4,900 m) was required to avoid summertime "chop" over the Rocky Mountains, and to allow the aircraft to fly around the thunderstorms that can sometimes block mountain passes, which meant a pressurized cabin would be the most comfortable for passengers on long flights".
So the very first reason to fly higher was simply to give a smoother ride to the passenger when flying around mountains. Less turbulence also implied that the airplane could fly faster - the first thing to do upon encountering an area of turbulence is reducing speed: With a cruising speed of 220 mph (350 km/h) at 20,000 ft, the Sratoliner was faster than comparable airplanes of the same era.

Quoted text from Wikipedia


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Where do I find the requirement for an AFM to be serial number specific?
It seems to be generally accepted that an AFM must be serial number specific to that aircraft it was made for and delivered with. I am having difficulty finding the reference for "serial number specific".

Is it just English? The AFM of an specific aircraft should be linked to the aircraft (as serial number, not just model, type, variant, year, line, etc.).

Yes, Understood the AFM should be linked. Is there regulatory directive stating as much?

Ok, so I think I misinterpreted your * I have found all the references that states such*, which make more sense

My apologies for the confusion. I will rephrase.

I'm puzzled.  If you have found "all the references" saying an AFM must be specific to an individual airframe, how could there be any doubt that it needs to be serial number specific since that is the means by which they are differentiated?

Thank you for the response Micheal I rephrased the question removing the "all" comment.

For the US, the FAA says in 25.1581-1 (Airplane Flight Manual):

The AFM may address either a single airplane model (i.e., hardware build) or several models of the same airplane type.

If information is provided for more than one model, the AFM should clearly identify which operating limitations, operating procedures, and performance information apply to each model (e.g., by model designation, serial number, etc.).

If the AFM format is such that different pages apply to different airplanes, the log of pages should clearly
identify the specific pages of the AFM that apply to each airplane.

Thus if the AFM provides specific information applying to a specific serial number, it must specify this number. Else it has to specify the appropriate level for which it is written, it can be the model or even the type. The serial number may appear only on pages which are serial number dependent, and not on the other pages.
It is frequent to see serial numbers or registration IDs specified on some pages of the manufacturer or operator FCOM.


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Understanding the constant speed prop and 2-4 degrees optimum
I've read through some answers on here and just wanted to write my own description of how this works and let someone check for me. Currently I've just started to fly an King air 100 with PT6A
We generally have 3 RPM settings to use. 2000RPM for takeoff, 1900RPM for climb, 1750 for cruise.
The CSU only knows the desired RPM to maintain on the propeller, (The RPM is measured from the tachometer on the 1st stage reduction gear box) as set by the pilot. The way is knows what desired RPM to maintain is via the position of the speeder spring.
At takeoff, we ensure the props are in fine and once we increase the power to the point the propeller comes out of the low pitch stop and is within the governing range, (should govern at 2000RPM and should not go above that, otherwise the CSU has failed but the prop should be protected by the OSG at 2100RPM) the CSU will now be able to change the blades angle.
At the start of the takeoff roll, our KTAS is very low, hence the horizontal compoent of velocity is low (Hv) and our blade angle will be basically at the low pitch stop in order to maintain the optimum angle of 2-4 degrees. Once we start to accelerate our KTAS increases, hence the Hv increases. Now as our Hv has increased the blade angle of attack has now decreased, The CSU has no clue the blade angle of attack has decreased but what it has sensed is the RPM slightly increasing above the set value of 2000RPM as the centrifugal force has increased causing the flyweights to move outwards. Now that the flyweights are moving outwards away from This pushes the speeder spring down and entering a overspeed condition. Now the checkvalve moves down and oil pressure decreases until the blade is back onto an onspeed condition, where the flyweights are no longer moving out and the angle of attack should be around 2-4 degrees again.
So the CSU will continue to constantly increase our blade angle as we acclerate and climb away. Now once in climb are TAS is considerbly higher, hence our Hv is considerbly higher. Now (heres my confusion) we reduce to climb RPM, down to 1900. We do this in order to maintain again the 2-4 degrees on the blade angle of attack. As we bring back the prop levers to 1900RPM what we are doing is changing the position of the speeder spring. We are moving it down, so the oil pressure decreases until the flyweights are no longer moving out?
We do this process again in cruise, but more significantly down to 1750 as the TAS increase from climb to cruise is greater than from inital takeoff speed to climb speed. Using the 1750RPM it should give 2-4 degrees angle of attack a the high TAS.
This is all to maintain propeller effiency. If we were to fly in cruise say at 1900 RPM our blade angle of attack would be too small and we would be getting drag (what type of drag?)
Note: Im also aware of wanting to favour max power over max efficency on takeoff and landing phases via Power= Thrust x RPM, hence we would get the most power with props fine. Do this mean the aircraft will accerlate the quickest with the highest power?
Thanks ,ollie

When we talk about "Power" there is engine output, or horsepower, as Torque × RPM and aircraft power, which is Thrust × Velocity.
If you want maximum thrust from a prop (at any airspeed), it's a combination of maximum (allowable) RPM and best angle of attack to the relative wind for the prop blade.
The best angle of attack of the prop blade, which is a spinning airfoil, will produce the highest Lift to Drag ratio, just like a wing.
If the angle of attack is not optimal, then the prop must absorb more power from the engine to generate the same amount of thrust.
If the engine can't give any more, then thrust will drop.
At any airspeed, with any throttle setting, the best prop AoA will produce the most thrust, therefor, the best acceleration.
The reason one changes from fine while climbing to coarse while cruising is the engine RPM slows down in cruise while airspeed is generally higher than in climb.  We can see the changes in prop RPM and airspeed will reduce prop AoA (to the relative wind) unless the blade is opened up a bit.

So thats why in a fixed pitch a/c in cruise, the prop is absorbing more power from the engine to generate the same amount of thrust, but in a constant speed it isn't absorbing more power, as the angle of attack is optimal as we have increased the blade angle. Does this increase fuel effeciency?

@OllieSeaward Yes! And also greatly increases the airspeed envelope that the aircraft can operate in.  This is why a constant speed/variable pitch prop can typically pull an aircraft of similar power much faster.  With fixed pitch one must choose between a "climb prop" or a "cruise prop".  The amount of power the engine puts out is proportional to fuel input, the amount of thrust produced from that power is proportional to how close one is to best AoA.

The spinning prop produces drag (resistance to engine power).  If the blade is too fine, the engine can overspeed (if it isn't governed).  At constant RPM, too fine means less thrust.  Too coarse means less thrust.  Proper blade pitch depends on relative wind (both RPM and airspeed).


text:
How to size a stratospheric hydrogen airship capable of supporting its own weight and 10kg of payload?
Is there a (simple) formula that tells me how to size a single lobe airship (volume, dimensions, max diameter) high-altitude platform and how much it should be inflated with hydrogen at sea level so that it climbs to 66,000 feet with 10kg of payload and stays there fully inflated? Do I account only for pressure change or are there other factors affecting this?

Study weather balloons.

One mol of helium has a mass of 4g, while a mol of nitrogen has a mass of 28g.  A mol has a volume of 22.4 litres at 0C, so a helium balloon has roughly 1g of buoyancy per litre of gas.  Once you’ve established a volume of gas is your balloon, the buoyancy doesn’t change with temperature or pressure provided that the air outside is at the same temperature and pressure as the helium inside.  For high altitude you’ll want your balloon to be full at your target altitude (you may be happy for it to burst at higher altitude or vent gas in a controlled manner) so at ground level it will be well under-inflated.  Helium tends to diffuse quickly through balloons but if your flight time is relatively short (a few hours) then you may not need to factor this in.


text:
Is it possible to find the actual dimensions of the Radial Drive Shaft and Bevel Gear of the CFM56-7B?
I am trying to draw to make a 3d model of the RDS in the CFM56-7B and I wasn't able to find the dimensions anywhere on the internet. And, what are the materials used to make them?

If you don't need engineering accuracy, you can start from this 2D blueprint, and knowing some dimensions, find the other.
If have done that to create small 3D models, and for the CFM56-7B, the measured result is this one (in Blender, not really a CAD application):

I think the scale is correct (the fan diameter is 61 in, 1.55 m). You can use the grid to evaluate other dimensions, the step is 1 cm. The shaft in context:

The inner and outer RDS are hollow and made of steel alloy.

Thank you for the answer. One more question, probably stupid but what's the diameter of the shaft?

@Nighthawk25, here are some more measurements (the golden shaft is N1 for reference), remember they are not more precise than the 2D blueprint used to model the parts.


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Do fighter jets have radio PTT button?
I know that non-fighter aircraft have a PTT button on the yoke for when the pilots need to transmit via radio. However, commercial aircraft usually have 2-handed yokes while jet fighters usually just has a "stick" which I imagine would make transmitting easier in commercial planes. Not to mention that commercial planes, under normal circumstances, also do not need to maneuver like fighters during combat maneuvers.
So, do fighter jets have a PTT button on the stick, or do the pilots wear something like a microphone that automatically picks up audio then transmits, or is it something else?

The commercial plane yoke/stick comparison isn’t as straightforward- Airbus, roughly half the passenger plane market, uses sticks.

The PTT mic switch on a fighter aircraft is typically on the throttle(s), actuated with the pilot's left thumb.  The control stick has plenty of buttons and switches, but they're typically used to control radar & other sensors, weapons delivery, and such things -- things that might need to be actuated at about the same time that a radio transmission needs to happen. So the usual convention is that the mic switch is about the only thing that the thumb of the left hand, on the throttle/s, is needed for.
Wikipedia provides this diagram of the switches on an F-16's stick and throttle:

In this case, the pilot's left thumb has a couple other controls it can manipulate, but they're sufficiently non-time-sensitive that it's not a problem being unable to manipulate them at the same instant as keying the mic.  Also, on this setup, the PTT works in two directions, pushing forward to talk on the VHF radio (perhaps to forces on the ground or to other aircraft in formation) and pushing the same switch backwards to talk on the UHF radio (perhaps to an AWACS or tanker aircraft).  (The diagram also shows "In" and "Out" positions, pushing that switch to the right or left. I'm not sure where those transmissions would go.)

this is a great answer! can you reco some links for further reading?

Wikipedia has a well-labeled diagram of the switches for an F-16; I'll see if others are better & edit something into the answer.


text:
Has any aircraft in history ever pulled 7g for 15 or more seconds?
According to this post "no aircraft has ever pulled 7g for two seconds"
If the interpretation of the video is correct, has any aircraft in history ever pulled 7g for fifteen seconds?

You quote some random person on the internet as fact... there are plenty of F-16 and F-18 pilots who would laugh at that statement & discuss routinely pulling, sustaining 7 or 8 or 9 G's for much longer than 2 seconds. Are you looking for those sorts of recollections to be documented? Or something else?

Yes, find one that said 15 seconds

Your question "Has any aircraft in history ever pulled 7g for two or more seconds?" is perfectly clear and simple to answer. No reason to close.  The rest of the text is based on a false assumption, frankly comes across as a bit unhinged, and I'm not even going to touch the link.  Don't need to... and you didn't ask anyone to verify it, although Chris seems to have done an excellent job debunking it.

Amusing that post: to uphold their theory that no aircraft can pull 7g they use a chart of an aircraft that can happily pull even 9g

@sophit The strangest part to me is the completely unsupported jump from "this aircraft only pulled 4g" to "no aircraft has ever pulled 7g for two seconds."

.. and the Energy maneuverability chart in the link clearly show a small region with positive specific Energy (Ps) above and to the right of the 7 G line.  That clearly indicates that this aircraft can pull 7 Gs indefinitely, (or until it runs out of fuel anyway).

Many aircraft are rated for 7Gs or more, and certainly many of them in history have pulled 7Gs for more than 2 seconds.  I know this to be a fact because I have personally experienced 7Gs for more than 2 seconds.
There is no problem with duration of time loaded at a certain G level.  For example, if an airplane is rated for 9Gs it can pull 7Gs for 2 seconds, 15 seconds, or several minutes.  The only real limit is fuel and pilot endurance.  (or pain threshold, 7Gs hurts...)
There are no time limits under load.  What ultimately leads to fatigue is the flexing caused by the number of cycles loading and unloading at various peak G loads.

As a non-pilot, I did the same on my F-16 incentive ride in Germany. In the prebrief, I asked my pilot...I want to see 9g on the meter. "OK, no problem". First attempt, 8.7. Second, 8.9, 3rd and last, 9.1 With me driving. Each was "more than 2 seconds".

@MichaelHall as you have noticed, the user that posted the original question keeps coming back with new accounts, as his original account has been suspended network-wide. His intensions are unclear to me, but he tries to solicit some kind of response. Please don't grace his behavior with any response, apart from flagging for moderator attention. As the saying goes: " don't feed the trolls".

The "elementary math" in the post you've linked to is completely wrong. The formula they used is only valid if the aircraft is in a level turn with a constant radius of curvature (even then, it's ignoring the vertical component of lift required). But also it's wrong even assuming all that.
In the video, the aircraft goes from about point 4 in the picture to point 5 in the picture in about 3 seconds. Assuming it takes a circular arc, the path something like as shown:

Note: the points simply don't lie on any circle. With so few points, trying to extrapolate the actual ground track is mostly guesswork. Exactly what circular arc you end up with depends on what points you try to include in it.
This is a 60 degree arc, with a radius of about 940 meters, giving an arc length of 984 meters. Assuming the arc takes 3 seconds, this gives a groundspeed of 328 m/s. The acceleration is then $(328{~\rm m/s})^2/940{~\rm m}\approx115{~\rm m/s^2}\approx 11.7~\rm g$. Note that tightening the turn, as appears to happen based on this person's analysis, requires even more g's than staying in a constant-radius turn.
Almost 12 g's, that's crazy! Literally crazy. This is obviously incorrect. The reason for this is probably that the poster of the question also got the locations marked incorrectly, and the airplane was in a greater radius turn than these two points imply. The calculated number of g's is extremely sensitive to the actual ground track, and attempting to estimate it based off looks out the window is difficult at best. It's also quite sensitive to the time- I estimated 3 seconds based on watching the aircraft's shadow, but if it's 4 seconds instead you end up with a more reasonable $6.6~\rm g$. It's definitely not five seconds- the building they label as point 4 is almost directly out the pilot's front window at 1:18 and they are well past the trees by 1:23.
The direct error in their math is to blithely assume that 1 to "somewhere between 7 and 8" is 180 degrees. Plainly 1 to 6 is approximately 180 degrees, while 1 to 8 is around 227 degrees. Despite the unsupported assertion that "if we optimistically asusme it got to (8), thats still under 7g," using the same radius of 1000 meters and 15 seconds but with 227 degrees instead gives a little over 7g.
In short, this video's displayed g-forces plausibly agree with the aircraft's actual flight path. Since the aircraft goes considerably over 7g for considerably more than 2 seconds, the answer to your question is "yes." It is doubtless only one of many times this has been done.


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What are the techniques that delay the formation of shock wave over wing?
In addition to winglets and other techniques like sweeping wing back what are the other techniques that delay the formation of local shock wave over the wing?

Welcome to Stack Exchange! Your question is a little confused, in the sense that you are asking two questions in one post. Could you please align the question title and question body?

Winglets are not used to delay shocks. They help to squeeze a bit more lift from a wing and reduce induced drag for a wing of given span.
Winglets help to increase the pressure difference between the top and bottom side of the wing near the tip. This leads to higher overspeeds and earlier shock formation. Note that in a well balanced design the first shocks appear at about the same speed over the whole wingspan, so adding a winglet will let shocks appear first near the tip and a slightly lower speed.
The prevailing techniques for delaying the formation of shocks are to:

fly more slowly. Especially for turns or pull-ups, speed should be reduced beforehand.
increase wing sweep.
adjust pressure distribution over chord such that suction peaks are avoided. This means that most lift will be created by rear loading, so a larger horizontal tail and more torsional stiffness of the wing are required.

To anonymous downvoter: Why did you downvote this answer?  Do you disagree with the statements contained in it?

I edited the title to reflect the body of the question, you might want to adjust your answer consequently. +1 from my side

Sorry if I'm not picking something up in the provided link, but what is meant by rear loading? Also +1 from me

@Wyatt Rear loading means litte camber in the thick forward part of an airfoil and high camber in the thinner, rear part. This shifts lift production especially at low angle of attack to the rear part of the chord.

In addition to the fine answers, supercritical airfoil, Richard Whitcomb, and Kawalki are some good search terms to learn more.

In addition to Peter Kämpf's fine answer, shocks can be weakened, delayed, or avoided by:

Reducing thickness of the wing.
Reducing lift coefficient (related to his turns and pull-ups comment)
Local area relieving where lifting surfaces meet non-lifting surfaces.  For example, coke-bottling a tip tank can prevent the superposition of two areas of super-velocities.

Should have thought of reducing thickness myself! +1

@PeterKämpf Sometimes we miss the obvious.


text:
Had there ever been civil aircrafts with more than four engines?
The Boeing 747 have four engines, the 737 two, and the 727 three. Most modern commercial jet airliners today have two engines. I know that some military aircrafts, (like the Boeing B-52 Stratofortress) do/did in fact have many more engines. However, I've never seen or heard of any commercial aircraft with 5 or more engines still being used today. So, there has ever been such an aircraft?

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

Depending on what you consider a civilian aircraft, the Antonov An-225 "Mriya" (https://en.wikipedia.org/wiki/Antonov_An-225_Mriya) also qualifies - it's definitely "civilian", as it was built for the Soviet space programme, was operated by a civilian operator and mostly used for civilian cargo after that, but I'm not sure if you had cargo aircraft in mind when you asked your question. It had six Progress D-18T turbofan engines (the same engines as its smaller sibling An-124, which had four of them).

image source

Thanks a lot for your great answer! It didn't realized that there were 6 engines on the Antonov An-225 "Mriya" before, since it's other features and aspects are also very intriguing. It is exactly what I'm looking for, since it's still flying just a few years ago. Sadly, it was destroyed by Russian forces on February 24, 2022 in the still on-going Russo-Ukrainian conflict. Anyways, your answer is great and is just what I'm looking for. Thanks!

Good stuff plagiarising my answer! :)

@Cloud: sorry, I only skimmed your answer and didn't see the reference to the An-225, honest! I think you should have started your answer with that rather than with the non-functioning fifth engines :)

Of course there was one. It had a total of twelve engines.

Dornier Do-X over Manhattan (picture source)
The largest airplane of its time, the Dornier Do-X was a flying boat with initially twelve Siemens Jupiter air-cooled radial engines which were later exchanged for Curtiss Conqueror water-cooled in-line engines. In the typical Dornier way practised since 1916, they were paired in push-pull twins and placed on top of the wing. Three planes of this type were built and two sold to Italy. The prototype D-1929 undertook a two year world tour to win customers, but except for the two planes for Italy, it was not successful.

In case anyone only counts 6 engines, each mount actually contains 2 engines, as they are aligned in a push-pull configuration with 1 prop facing aft, and the other fore.

Thanks for your answer, it is very informative. The aircraft does indeed have 12 engines and is an civilian aircraft. However, it was built and used nearly a hundred years ago, so it  isn't exactly what I'm looking for, which is aircraft that is still used today. Still, thanks for your great answer, I learned something new because of it.

@EndAntisemiticHate Thanks for your explanation, I originally though it only had 6 engines, but your comment helped me figure out that there was in fact 12 engines. Again, thanks for helping me figure that out!

to be clear, your original question did not contain any wording about "being used today". it's a moving target.

The Spruce Goose had 8 radial piston engines each driving a single propeller, enough to lift it out of the water on its single ground effect flight in 1947.

The Bristol Brabazon Mk1 had 8 piston engines, each driving a propeller. The 4 nacelles contained 2 counter-rotating props each. The aeroplane first flew in 1948, was then to be upgraded to Bristol Proteus turbo-props, and then was cancelled before any airline ordered the plane.

The Saunders-Roe SR.45 Princess had 10 Bristol Proteus turbo-prop engines, each driving a propeller, and was developed as a flying boat to transport passengers in great intercontinental luxury. Four of the nacelles have twin counter-rotating props, the outer nacelles drive a single prop. The design was outdated by the time it flew in 1952.

And yeah, military aeroplanes.
The B-36 Peacemaker had 6 piston powered propeller engines when it first flew in 1946, plus two pods with 2 turbojets each added in , for a total of 10 engines.

The B-52 bomber had 8 turbojet engines, developed in a time before turbofans could comfortably do the same job. The Wiki article shows an upgraded B-52H, with 4 turbofans, using lots and lots of less fuel compared with the 8 turbojets.

Spruce Goose was military as well wasn't it?

@OrganicMarble well...financed by the military during WWII, but intended as a group transport...so functionally it would have been a passenger plane after the war. Was my consideration.

Tupolev ANT-14 had 5 engines, Tupolev ANT-20 had 8 engines, and Tupolev ANT-20bis had 6.  All were civil airliners/cargo aircraft.
The Italian Caproni Ca 60 Transaereo had 8 engines and first flew in 1921 (though it never went into service and crashed on its second flight).  I'm certain there were others in the 1930s.

Can you please provide some links for the aircraft? Because I'm having a hard time googling every one of them.

Never mind. I googled every one of them and found their Wikipedia pages. However, it seems like all of those aircraft were in service nearly 100 years ago, and retired before or during World War II. So this is not exactly what I'm looking for. Still, thanks for your effort and help!

Not really. The 747-100 and 200 had:

For transportation of spare engines, the 747 can accommodate a
non-functioning fifth-pod engine under the aircraft's port wing
between the inner functioning engine and the fuselage.

I don't know if you're including transport aircraft, if so the Antonov An-225 Mriya had six engines.
So unless you count those, no*.
*You asked for "civil aircraft", so I'm excluding military types.


text:
What's the optimal taper ratio for a commerical short/medium haul aircraft?
I'm designing an R/C plane with interchangeable winglets and I've so far calculated the taper ratio to be .246 (aspect ratio 9.5 if that helps). I'm taking some design/geometry cues from the A320. How does taper ratio affect aerodynamics?

Taper ratio is mainly used to control how lift is distributed spanwise with the goal of having an elliptical distribution in order to reduce induced drag. The following plot represents how lift changes spanwise according to the taper ratio (source):

As a rule of thumb, taper ratios of 0.25 and 0.45 are the optimal for  swept and unswept wings respectively.

The lift distribution and the effect of taper is also strongly influenced by the sweep on the wing.  This figure appears to be for an un-swept wing, so use caution when applying to something with the ~35deg of sweep of a commercial transport.

Actually, two questions are asked here and a third is what are your requirements, really? Sounds like you want the aesthetics of a 'real' short/medium commercial a/c but it will be operated at low speed and be of small scale.

@AeroAndy I'm looking to replicate the aesthetics of a commercial airliner as a testbed for most categories of winglets out there. (honestly this sounds kinda dumb now since most winglet effects are realized in cruise) but anyway

Don't be hard on yourself. :)  A low-speed comparison of different winglet geometries would be interesting and relevant. That said, it begs the question of the ability to discern configuration differences from your flight test data. Also, be aware of Reynolds # effects. For example, depending on the scale & speed of your R/C platform, the winglet may have extensive laminar flow whereas full-scale will have a predominantly turbulent boundary layer (in the absence of specific attempts otherwise). There are methods to mitigate this.

In addition to aerodynamic performance, I'm betting structural dynamics, loads, mass properties and producibility are some other key areas driving winglet design.


text:
What is the pole on the right side of the A4E Early?
I was playing War Thunder, and I wondered: what is the pole on the right side of the A4-E Early (picture source)?

Screencap of what you're seeing? But assuming you're talking about a Navy A-4, that is likely the air refueling probe.

I can only guess the refueling probe is correct, but who knows that OP means for sure: https://wiki.warthunder.com/A-4E_Early#Media

Similar to What is this nose projection on the Tupolev Tu-95?, or What is this tube along the right side of a Blue Angel? and many other.

This pic captured from:
https://wiki.warthunder.com/A-4E_Early_(M)_(Israel)

The "pole" on the right side is the air refueling probe.


text:
Can anyone identify the airplane in this 1920s/30s photograph?
Can anyone identity the aircraft in this photograph (taken late 1920s or early 1930s, I think)?  It belonged to my great-grandfather, who lived near Milwaukee, Wisconsin, however, I have no idea where the photograph itself was taken.  The plane seems similar to a Stinson SM-8.

You are correct; it's a Stinson Junior. The engine in the old pic is definitely a Wright Whirlwind, based on the exhaust port orientation and the valve covers, and having 7 cylinders, means it's probably a Whirlwind J-6-7 760 cubic inch radial.  That means it's likely a Junior SM-8B, or possibly an SM-2AC, both of which had that engine.

Thank you for your help.  I couldn’t figure out why the engine seemed different from other photographs of that model, and was confused by the lack of a window behind the door (again, what I’ve seen in other photographs).


text:
Do these sound like plausible ATC communications (specifically in China, if that's relevant)?
Working on a small translation project that involves civil aviation. The setting is a Chinese airport. Same idea as my other question -
Do these seem like plausible radio communication dialogue between ATCs and pilots? (Bold font indicates an area I'm particularly unsure about.)

(ATC to pilot) [FLIGHT #], [PLACE] Tower. Looks like you had a tire burst. Please confirm.
(ATC to ground) Runway 17L, [FLIGHT #]'s got a tire burst, have emergency vehicles ready. Runway is suspended.
(Pilot to ATC) [PLACE] Approach, good evening. [FLIGHT #], with you at 4500 meters, information H. Airspeed 300 knots, request ILS approach runway 17L.
(ATC back to pilot) [FLIGHT #], we’ve got a situation on the ground. Runway 17L is not available. Descend and maintain at 3500 (meters).
(Pilot back to ATC) Can we use runway 04? We can accept yawing. [FLIGHT #].

For (3) I've pretty much done a direct translation from the Chinese. I couldn't find anything online on how exactly altitude and airspeed are reported.
Again, here are some short captioned ATC recordings from Chinese airports that I've been taking a look at:

https://www.bilibili.com/video/BV1Cs411V7gt/?spm_id_from=333.788.recommend_more_video.0&vd_source=6573b5aa3eb27ecb2432df9114663b88 (Shanghai Approach, see for example at around 1:27)
https://www.bilibili.com/video/BV1MW411g7D2/?spm_id_from=333.788.recommend_more_video.4&vd_source=6573b5aa3eb27ecb2432df9114663b88 (Beijing Approach, a decent amount of communication in this one is done in English)

(I've been getting the sense that the conventions for radio communications might be somewhat different in China? At least the units used for altitude appear to be meters.)
Thank you!

Units of altitude are indeed meters in China and some other Asian countries. And Russia has somewhat recently switched to feet-based flight levels, but keeps using meters below transition.
Units of speed should be kilometers per hour too, but I guess in that case the controllers will work with either.
In 5 I think the correct English term (I understand you translated this from Chinese, right?) should be circling. A “circling approach” means that the aircraft would continue the approach to 17L until they are clear of clouds and have good visibility of the airport and then proceed to visually fly around the airport to land on runway 4. That is a defined procedure, approach plates list weather minima needed for it, but some airlines don't have it in their standard operating procedures so they don't have to train it and thus couldn't accept it. So the pilots are notifying the controller that they can accept it—and suggesting it would be a reasonable thing to do at this point.

The Chinese term is 偏航, and the English term I found for it was "yawing" (https://baike.baidu.hk/item/%E5%81%8F%E8%88%AA/9517125). But your explanation makes sense and it's also possible that the author of the text in Chinese (a work of fiction) may have misused the term.

@Pearl6527 It is quite likely it is the correct term for that kind of procedure in Chinese. The meaning is close enough and technical terms often don't match dictionary translation. It can even mean yawing (as in changing heading) in general aviation context, but still name the specific kind of procedure that in English is called a circling approach. You can try checking a Chinese pilot/controller glossary if you want to be sure.

I see, thank you!